The invention is situated in the domain of chemical sensors for the analytic characterization of gases and liquids.
The invention concerns the use of semi-conductive coatings, based on doped xcfx80-conjugated arylene alkenylene oligomers, in the active layer of sensors, and in arrays of such sensors applicable for the analytical characterization of gases and liquids, applicable in detecting systems for chromatography as well as in the context of an xe2x80x9celectronic nosexe2x80x9d or xe2x80x9celectronic tonguexe2x80x9d.
High-performance liquid chromatography (HPLC) is an analytical quantification and detection method in which an eluent is forced through a packed column. The mixture containing the product that has to be detected or quantified is submitted to an eluent flow. Since different products have different elution speeds, the products are separated from each other at the end of the column. After being separated on the column, the product is detected in a flow-through detector. The detector response can be translated into the concentration of the product.
Similarly, in gas chromatography, detectors are used to quantify or detect analytes in a gaseous mixture. Detectors that can be used for these purposes can be the usual electrochemical detectors (ECD). The operation principles of ECDs are well described.
ECDs operate on the basis of potentiometric, amperometric and conductimetric phenomena. The present state of the art will be reviewed for such systems.
Potentiometric detectors are widely used in analytic chemistry, the best known example being the pH-meter. The operating principles are well known. Such detectors are often specific for one ion (ion-selective electrodes). In chromatographic systems (and other systems, based on hydrodynamic measurements) however, potentiometric detection is still poorly developed. For these applications one needs sensors with low specificity, in contrast to batch techniques in which high specificity is required.
The kind of electrodes that are mostly used currently are metal electrodes (copper), so-called liquid-membrane ion-selective electrodes, Ag+/AgCl electrodes, and anion-exchange membranes (Potentiometrische detektie van anionen in LC en CE met polymere vloeibaar-membraan elektroden, B. De Backer, Universitaire Instelling Antwerpen, 1995). Such potentiometric sensors are not yet commercially available for liquid chromatographic systems. This is due to the low sensitivity, the slow response and the sometimes irreversible behavior of the sensors under chromatographic conditions.
Liquid membrane electrodes work well under chromatographic conditions as well as in Capillary Zone Electrophoretic (CZE) separations (Grate, M. H. Abraham, Sensors and Actuators B, 3 (1991), 85).
In HPLC one usually uses glassy carbon as the active material in amperometric detectors. Glassy carbon made amperometric detection in HPLC a success in the last 20 years. A wide range of products can be detected with this material. Yet, many chemical compounds can in practice not be oxidized or reduced on the surface of glassy carbon, because of the low reaction rates. Each year about 100 publications report about efforts to modify glassy carbon while improving the kinetics of the oxidation and reduction processes. So far, no significant improvements were made. Another material successfully used in amperometric HPLC detection is polycrystalline platinum, used in the so-called xe2x80x9cPulsed amperometric detectionxe2x80x9d (PAD) of carbohydrates. Recently, much research is carried out on enzyme-electrodes. The enzymes, immobilized on the electrodes, act as catalysts in the electron transfer from the analyte to the electrode underneath.
Glassy carbon is chemically inert, thereby making developments such as electrode derivatisation or enzyme coupling difficult.
A different application for chemical sensors is in the context of an electronic nose. An electronic nose is an instrument in which an array of sensors, each of which have partial selectivity, is used in combination with a pattern recognition system, and which can be used to recognize simple and/or complex aromas and gas mixtures.
Such an array of sensors is described in U.S. Pat. No. 5,571,401.
The electronic nose can be employed in a wide range of applications, for example in industry, medicine, environmental protection, distribution and transport as well as in forensic investigations. In the food and beverage industry one may apply a chemical nose to monitor the freshness, to control quality of the starting, and the middle, and the final products as well as to monitor fermentation processes.
The advantage is that the quasi on-line (in situ) monitoring of the aroma (odor) may lead to an automated control system. Identification and/or quality control of starting products are potential applications in the chemical and pharmaceutical industry, because the xe2x80x9cnosexe2x80x9d offers a fast and universal analytical method. In medicine, the nose can be used in breath analyses. The patient""s breath allows the diagnosis of some diseases, for example the odor of acetone is an indication for diabetes. The odor pattern may be helpful to identify the source of industrial emissions. Yet another potential use is a fast risk analysis in the event of road accidents involving chemicals.
On the spot determinations of fire accelerators may be another application. This list of possible applications is not complete, and other applications will appear. It shows, nevertheless, that the development of the electronic nose is important with many advantageous applications. In several of these applications, a small response time is needed.
The nose must be able xe2x80x94after a learning stagexe2x80x94to distinguish between the aromas of two or more mixtures, each containing a multitude of components. The sensors must operate on a molecular level. Furthermore, a sensor in an electronic nose must have partial selectivities to a wide spectrum of gases, rather than a high sensitivity to only one particular gas. Most existing gas sensors lack the wide spectrum of sensitivities to a variety of gases. Sensors based on metal oxide semi-conductors (MOS) Type 1 sensors, are widely and successfully employed in the sensor industry. Yet in the context of a chemical nose they are inferior compared to other materials. Particularly the number of different MOS materials that can be produced by inorganic synthesis, is very small compared to variability offered in products through organic synthesis.
Other types of sensors considered for application in an electronic nose are all based on the partial, selective permeability of polymers for gasses. On this single physical phenomenon, many technologies can be grafted, such as electrochemical sensors. Also, one knows surface acoustic wave and quartz crystal microbalance sensors using polymer coatings (Dickinson, J. White, J. S. Kauer, D. R. Walt, Nature, 382 (1996), 697) (type 2 sensors). Type 3 sensors exist, based on the fluorescent properties of optical fibers with a polymer coating. Type 4 sensors are conductimetric sensors based on conventional polymers where conductivity is introduced by mixing graphite particles with the polymer.
Conductimetric gas sensors based on conducting polymers form a group of sensors (type 5) that are important in the context of the electronic nose (Persaud, G. Dodd, Nature, 229 (1982), 352). Conducting polymers consist of a long sequence of alternating single and double bonds. This xcfx80-conjugated system can be made conductive via oxidation or reduction in a process called doping. The conductivity is influenced by the environment. In other words, conductivity changes upon contact with different vapours. These compounds are well suited for implementation in a chemical nose.
The use of polypyrrole, polyaniline, poly-N-methylpyrrole and poly-5-carboxyindole has been reported. (P. N. Bartlett, S. K. Ling-Chung, Sensors and Actuators, 20. (1989), 287). More type 5 sensors have been made by variation of functional groups on the main skeleton or by a variation of doping materials.
At this moment in time 5 companies offer commercial versions of an electronic nose. They are
(1) Aromascan plc., Electra House, Electra Way, CREWE CW1 1 WZ, UK (offering a nose based solely on conducting polymers);
(2) Neotronics, Western House 2, Cambridge Road, Stansted Mountfitchet, Essex CM24 8BZ, UK (offering a nose containing conducting polymer and MOS sensors);
(3) AlphaMOS s. a., 3 Avenue Didier Durat, 31400 Toulouse, France (offering a nose solely based on MOS sensors);
(4) Nordic Sensor Technologies AB, Teknikringen 8, S-583 30 Linkxc3x6ping, Sweden (offering a nose based on MOS sensors) and
(5) Lennartz Electronic, Bismarckstrasse 136, D-72072 Txc3xcbingen, Germany (offering a nose based on MOS and quartz crystal microbalance sensors).
Other references to this subject are:
ref. 1: New Scientist, February 1993, 18.
ref. 2: CandEN, August 1996, 31.
ref. 3: Bartlett, J. W. Gardner, xe2x80x9cSensors and Sensory Systems for an Electronic Nosexe2x80x9d, Kluwer Academic Publisher, Dordrecht, 1992.
ref. 4: Stetter et al., xe2x80x9cSensor systems for an electronic nose.xe2x80x9d, Kluwer Academic Publischer, Dordrecht, 1992).
ref. 5: Mills, F. Walsh, I. Whyte, Chemical Technology Europe, July-August 1996, 26
ref. 6: xe2x80x9cSelective sample handling and detection in high-performance liquid chromatographyxe2x80x9d, Journal of Chromatography Library, vol. 39 A, R. W. Frei and K. Zech, Ed., Elsevier, Amsterdam 1981.
ref. 7: xe2x80x9cPrinciples of ion-selective electrodes and membrane transportxe2x80x9d, W. E. Morf, Ed., Elsevier, Amsterdam 1981.
This invention regards chemically sensitive sensors, suitable for detecting analytes in fluids (in gaseous or liquid phase), characterised in that the chemically sensitive sensors comprise a chemically sensitive probe which comprises one or a blend of several arylene alkenylene oligoamrs. The arylene alkenylene determines at least one response signal. The invention further comprises means for converting the response signal to a sensor response. Said arylene alkenylane determines at least one response signal. The invention further comprises means for converting said response signal to a sensor response.
These oligomers can also be mixed with polymers. Doping is preferred to obtain workable resistances. Sensors according to the invention are applicable in a chromatography apparatus such as a GC (gas chromatograph) or HPLC (High Performance Liquid Chromatography) for detecting analytes. The possibility to use different arylene alkenylene oligomers for different sensors, which results in a different selectivity and sensitivity of said sensors for a given analyte, makes the sensors according to the invention very suitable for arrays of sensors. Combined with a pattern recognition system, such arrays of sensors are configurable as an electronic nose or tongue.
We recall that the electronic nose consists of a pattern recognition system in combination with a sensor array.
In analogy to the electronic nose, an electronic tongue can be developed: it would be an apparatus consisting of an array of sensors capable of characterizing the composition of a liquid. Again a number of sensor types come into consideration among which sensors based on conducting oligomers.
The response signal can be determined by the interaction of the oligomer with the analyte or the fluid.
Other systems that require several sensors with different responses can also be developed using sensors according to the present invention.
The response signal can be determined by the interaction of the oligomer with the analyte or the fluid.
The response signal can be determined by the interaction of the oligomer with the analyte or the fluid.